In recent years, hybrid vehicles and electric vehicles are gaining attention as environmentally friendly vehicles. A hybrid vehicle has, as a source of motive force, a conventional engine and in addition thereto a motor driven by a direct current power supply via an inverter. More specifically, the engine is driven to obtain a source of motive force and the direct current power supply also provides direct current voltage which is in turn converted by the inverter to alternate current voltage employed to rotate the motor to obtain a source of motive force.
An electric vehicle is a vehicle having as a source of motive force a motor driven by a direct current power supply via an inverter.
In such a hybrid vehicle or electric vehicle when a drive motor normally rotates an alternate current flows through each switching element of the inverter. However, if the vehicle has been ditched or the like and the motor has its rotor locked and thus stopped or rotates at a significantly low speed, a large direct current flows through a particular switching element and the element's heat loss will rapidly increase.
Means for preventing such overload on an inverter that is caused when a motor is locked or rotates at a significantly low speed have conventionally been studied. One such means is a technique limiting a torque that a motor outputs to limit an amount of a current flowing through a switching element of an inverter (see Japanese Patent Laying-Open Nos. 11-122703, 7-322401, 8-191503, and 2005-45863, for example).
However, if the drive motor is subjected to such output torque limitation and an immediately decreased output torque is provided then the vehicle cannot obtain force driving it, as desired, and cannot escape from a locked state or from rotating at a significantly low speed. For example if the vehicle is climbing uphill and the driver for example depresses the accelerator pedal in an insufficient amount, and the drive motor is thus locked, providing a limited output torque may cause the vehicle to fall downhill.
Accordingly for example Japanese Patent Laying-Open No. 11-122703 discloses an overload prevention device for an electric vehicle that contemplates preventing overload on an inverter without rapidly decreasing a torque output from a motor.
According to Japanese Patent Laying-Open No. 11-122703 the overload prevention device for an electric vehicle includes: temperature estimation means estimating the temperature of a bonded portion of each switching element from the value of the temperature of the switching element as detected; torque limitation value calculation means calculating a torque limitation value in accordance with the value of the temperature of a bonded portion of a conducting switching element as estimated; and control value limitation means limiting a torque control value by the torque limitation value when a motor rotates at a speed having a value, as detected, smaller than a predetermined value and the switching element's bonded portion has a temperature, as estimated, exceeding a predetermined value.
The torque limitation value calculation means identifies a conducting switching element from the position of a magnetic pole of the motor and calculates a torque limitation value in accordance with the value as estimated of the temperature of the bonded portion the conducting switching element, and when the control value limitation means limits the torque control value to at most the torque limitation value the conducting switching element's current is controlled to be a current output in accordance with the control value after the torque limitation.
Thus, when such torque limitation provides a decreased output torque, and the motor's conducting phase, i.e., the conducting switching element shifts to a switching element of an adjacent phase, a torque limitation value is calculated for the switching element that is now the conducting switching element. As the switching element that is now the conducting switching element has so far not conducted, its bonded portion has low temperature. Accordingly the torque limitation value therefor is high. Accordingly the output torque now increases.
Thus a torque control value is limited for each switching element in accordance with its bonded portion's temperature. As such if the motor is locked a rapidly decreased torque is not output. This can prevent the vehicle's occupant(s) from feeling uncomfortable.
As described in Japanese Patent Laying-Open No. 11-122703, however, the torque limitation value calculation means and the control value limitation means exert control on a precondition that the motor's magnetic pole positionally varies, i.e., the motor rotates. As such, if the motor's rotation is completely locked, a torque control value is necessitated to simply decrease from a maximal torque that is obtained when the inverter has passing therethrough a maximal current that it can pass, to a torque output from the motor as a maximal direct current that a particular switching element of the inverter can continuously tolerate is passed when the motor is locked (hereinafter the latter torque will also be referred to as a torque that can be continuously output). Thus, when the motor is locked, a rapidly decreased output torque will be provided, and a possibility arises that motive force required to escape from the locked state cannot be ensured.
Furthermore, the torque limitation value calculation means configured to identify a conducting switching element from the position of a magnetic pole of the motor and calculate a torque limitation value in accordance with the value as estimated of the temperature of a bonded portion the identified switching element, contributes to complicated contents for control. Furthermore, the torque is generated with a magnitude varying with the position of the magnetic pole of the motor, and it is still unavoidable that the driver of the vehicle feels uncomfortable.
The present invention has been made to overcome such disadvantage and it contemplates a motor drive device that can both prevent overload on an inverter and also ensure dynamic performance.